This short review focuses on the physicochemical properties of core–shell particles and their resulting chromatographic efficiencies as they relate to the comparison of their mass transfer kinetics to those of columns packed with sub-2-μm fully porous particles.
In recent years, column technologies have diversified for a multitude of separation applications. The improvement in separation efficiency attainable today on modern liquid chromatography (LC) columns results from the evolution of new packing materials and sorbents that offer analysts the choice to use fully porous particles, core–shell particles or silica–polymer monolithic columns, depending on the application (1). Among the new column packing materials, core–shell particles, also called superficially porous particles, of sub-3-μm diameter have gained considerable attention because of significant improvements in separation efficiency. Columns packed with core–shell particles can generate a minimum plate height below H ~ 3.5 (2,3) for small molecules, a similar efficiency to that of columns packed with fully porous sub-2-μm particles.
The use of core–shell particles as column packing materials is not new; in fact, they have been in existence and have been used since the early days of high performance LC (HPLC) (4–6). The main purpose of pellicular particles in the early days of HPLC was to improve the mass transfer resistance of large molecules to facilitate faster separation (6). However, pellicular particles later met their demise because such particles were not as robust and refined as the fully porous particles that were introduced later. For detailed historical accounts of the development of various pellicular particles as packing materials for HPLC columns, readers should refer to reference 7. The new-generation core–shell particles display significant improvements in important physicochemical parameters, such as particle-size distribution; thickness of the porous shell, providing substantial increase in surface area for retention (8); surface roughness or smoothness; and the purity and robustness of the base core–shell silica. A number of current manufacturers and academic researchers are currently investing a great deal of time in the preparation of core–shell silica particles (9); a list appears in Table 2 in the Majors article in this issue (10). The new-generation core–shell particles have been solely designed to compete with the sub-2-μm fully porous particles by providing a substantial increase in column permeability; this results in faster separations while maintaining the same high column efficiencies. Recently, another set of core–shell particles (Phenomenex's Aeris core–shell silica particle) has been released and is claimed to be a rival to the sub-2-μm fully porous particles for separating large molecules (11). This short review will summarize the key physiochemical properties of the new generation of core–shell particles that results in superior performance of HPLC columns.